1. Field of the Invention
The present invention relates to an electrolyzed water of anode side having a dismutation activity for superoxide radicals, as well as to a process for producing such an electrolyzed water. More particularly, the present invention relates to a process for producing an electrolyzed water of anode side by electrolyzing an aqueous electrolytic solution containing only ascorbic acid as an electrolytic aid and taking out the electrolyzed water generated at the anode side, as well as to an electrolyzed water of anode side produced by such a process, having a dismutation activity for superoxide radicals.
2. Description of the Related Art
A technique is well known in which a dilute aqueous solution of an electrolyte such as alkali metal chloride or the like is electrolyzed in an electrolytic cell comprising inactive electrodes made of platinum, a platinum alloy or the like and a separating membrane placed between the electrodes and the electrolyzed water of low pH (acidic water) generated at the anode side is taken out to utilize it for sterilization or disinfection. As the separating membrane, there is used a charged membrane (an ion exchange resin film) or a non-charged membrane of porous structure.
The electrolyzed water of anode side contains hypochlorous acid therein. In utilization of the electrolyzed water of anode side for sterilization or disinfection, the strong oxidizing action and chlorinating action of hypochlorous acid is utilized. Such utilization of the electrolyzed water of anode side is widely conducted in medical institutions, etc. Further, the ozone and dissolved oxygen present in a small amount in the electrolyzed water of anode side promotes granulation; therefore, the electrolyzed water of anode side is being studied as an aid in surgical treatment.
Meanwhile, the electrolyzed water of cathode side (alkaline water) generated at the cathode side can be obtained also by electrolyzing a tap water in place of the above-mentioned dilute aqueous solution of an electrolyte. The electrolyzed water of cathode side has been utilized for drinking, etc. In U.S. Pat. No. 5,736,027, there is disclosed other technique for producing an electrolyzed water, which comprises electrolyzing the above-mentioned aqueous electrolytic solution or a tap water by adding an organic acid (e.g. ascorbic acid or gallic acid) as an additive and not as an electrolytic aid.
In this technique, ascorbic acid is used in the presence of an electrolytic aid. The purpose of adding ascorbic acid is to (a) control the pH of electrolyzed water of cathode side and (b) remove the free chlorine in electrolyzed water of anode side.
Incidentally, the well-known Kolbe reaction (this has no direct connection with the above technique) is a reaction in which an organic acid (e.g. a carboxylic acid) is electrolyzed to release carbon dioxide and also generate a compound wherein two organic acid residues are combined, at the anode side. For example, there has long been known a technique in which citric acid is generated from acetic acid according to the Kolbe reaction.
Ascorbic acid has OH group at the 2-position and 3-position of the molecule. In an acidic state, the xe2x80x94OH of the 3-position is dissociated into xe2x80x94Oxe2x88x92 and H+, showing an acidity. In an alkaline state, the xe2x80x94OH of the 2-position is dissociated into xe2x80x94Oxe2x88x92 and H+. However, the degree of this dissociation is low and, therefore, ascorbic acid has never been used as an electrolytic aid. Incidentally, the electrolysis mechanism of aqueous ascorbic acid solution is complicated and the intermediates generated in electrolysis have not been specified; however, the mechanism is recognized to be basically a redox reaction.
Ascorbic acid is a strong reducing agent by itself. It is well known that ascorbic acid undergoes autoxidation in an aqueous solution and comes to have a lower reducing power. Ascorbic acid undergoes autoxidation generally according to the following reaction scheme (1). 
In the above, AsA, MDA, DHA and DKG refer to ascorbic acid, monodehydroascorbic acid, dehydroascorbic acid and 2,3-diketogulonic acid, respectively.
In recent years, it has been made clear that ascorbic acid allows superoxide radicals (this is well-known as a reactive oxygen) to dismutate and disappear. Therefore, ascorbic acid has come to draw attention as an antioxidant.
The dismutation reaction of superoxide radicals are expressed by the following formula (2).
Oxe2x88x922xe2x80xa2+Oxe2x88x922xe2x80xa2+2H+xe2x86x92H2O2+O2xe2x80x83xe2x80x83(2)
That is, superoxide radicals disappear and hydrogen peroxide is generated.
As described above, ascorbic acid has an action of allowing superoxide to disappear. However, there has hitherto been no report regarding an electrolyzed water of anode side having a dismutation activity for superoxide radicals, obtained by electrolysis of an aqueous ascorbic acid solution.
The present inventor paid attention to the ascorbic acid""s dismutation activity for superoxide radicals and made a study to obtain an electrolyzed water of anode side having the above action. As a result, the present inventor found out that an electrolyzed water of anode side having a dismutation activity for superoxide radicals can be obtained by electrolyzing an aqueous solution containing only ascorbic acid in a relatively low concentration without using any inorganic electrolyte (e.g. a water-soluble metal salt) as an electrolytic aid.
The present invention has been completed based on the above finding and aims at providing an electrolyzed water of anode side having a dismutation activity for superoxide radicals.
The present invention lies in the following [1] to [5].
[1] An electrolyzed water of anode side containing less than 0.1 mM of a water-soluble inorganic salt, 1 to 50 mM of ascorbic acid and 8 to 15 mg/l of dissolved oxygen and having a dismutation activity for superoxide radicals.
[2] A process for producing an electrolyzed water of anode side having a dismutation activity for superoxide radicals, which comprises electrolyzing an aqueous electrolytic solution containing less than 0.1 mM of a water-soluble inorganic salt and 1 to 50 mM of ascorbic acid and then taking out the electrolyzed water of anode side generated.
[3] A process for producing an electrolyzed water of anode side having a dismutation activity for superoxide radicals, according to the above [2], wherein the electrolysis is conducted using an electrolytic cell having a separating membrane.
[4] A process for producing an electrolyzed water of anode side having a dismutation activity for superoxide radicals, according to the above [2], wherein the electrolysis is conducted at a current density of 0.003 to 0.03 A/cm2.
[5] A process for producing an electrolyzed water of anode side having a dismutation activity for superoxide radicals, according to the above [2], wherein an aqueous electrolytic solution containing less than 0.1 mM of a water-soluble inorganic salt and 1 to 50 mM of ascorbic acid is fed into a continuous flowing type electrolytic cell having a separating membrane, at a flow rate of 500 to 3,000 ml/min and is electrolyzed continuously at a current density of 0.003 to 0.03 A/cm2.
When an aqueous electrolytic solution is electrolyzed in order to utilize the obtained electrolyzed water for drinking, sterilization and disinfection, there is used, as the aqueous electrolytic solution, a tap water or an aqueous solution obtained by adding to, a tap water, a water-soluble inorganic salt such as sodium chloride, potassium chloride or the like. In the case of a tap water, a certain amount of inorganic salts are present therein and they act as an electrolyte.
Meanwhile, there are reports (e.g. Japanese Patent Application laid open 11-33552) on a technique of electrolyzing an electrolyte- and ascorbic acid-added aqueous solution to produce an electrolyzed water. In this technique, ascorbic acid is used as an additive for removal of alkalinity. There is also known a technique (Japanese Patent Application laid open 8-229563) of electrolyzing an aqueous electrolytic solution containing ascorbic acid as an additive and reducing the hypochlorous acid present in the electrolyzed water of anode side generated, with ascorbic acid to suppress the generation of free chlorine in the electrolyzed water.
In all of these techniques, the aqueous electrolytic solution used contains an electrolyte (consisting of a water-soluble inorganic salt) as an electrolytic aid and, in addition, ascorbic acid. This ascorbic acid is used as an additive not directly associated with electrolysis.
When an aqueous electrolytic solution containing an electrolyte (e.g. sodium chloride) as an electrolytic aid and ascorbic acid as an additive is electrolyzed, the following reactions take place generally.
1. Cathode side 
In the above, AsANa refers to sodium ascorbate. In the electrolyzed water of cathode side, the H+ of the 3-position of ascorbic acid is substituted with sodium ion, whereby ascorbic acid becomes a sodium salt. As a result, only the H+ of the 2-position can have a dismutation activity for superoxide radicals. Thus, the dismutation activity of ascorbic acid is reduced to half.
2. Anode side 
In the anode side, as shown in the above formula (9), both the H+ of the 2-posiiton and the H+ of the 3-posiiton, of ascorbic acid disappear owing to the chlorinating and oxidizing actions of hypochlorous acid; as a result, there is obtained no dismutation activity for superoxide radicals.
The above is electrolytic reactions occurring at two electrodes when an aqueous electrolytic solution containing an electrolyte as an electrolytic aid and ascorbic acid as an additive is electrolyzed. In this case, a dismutation activity for superoxide radicals is obtained only at the cathode side; however, the dismutation activity is reduced by the formation of sodium ascorbate.
In the anode side, as mentioned previously, both the H+ of the 2-position and the H+ of the 3-position, of ascorbic acid disappear owing to the chlorinating and oxidizing actions of hypochlorous acid; as a result, there is obtained no dismutation activity for superoxide radicals.
Meanwhile, when an aqueous electrolytic solution containing only ascorbic acid as an electrolyte is electrolyzed, the following reactions take place at two electrodes.
1. Cathode side 
As shown in the formula (11), electrolysis allows the 2-position and 3-position hydrogen atoms of ascorbic acid to give rise to an electrophilic reaction on the cathode, whereby hydrogen gas is generated at the cathode side. Since the DHA generated by electrolysis is unable to release H+, the electrolyzed water of cathode side has no dismutation activity for superoxide radicals. Further, since ascorbic acid anion is transferred to the anode side, there is no dismutation activity for superoxide radicals by ascorbic acid anion, either.
2. Anode side 
As shown in the formula (13), ascorbic acid anion reacts with the H+ generated by anode oxidation of water and becomes ascorbic acid. Meanwhile, as shown in the formula (14), the oxygen gas generated by anode oxidation of water is consumed by oxidizing ascorbic acid into 2,3-diketogulonic acid (DKG) and, as a result, the dissolved oxygen amount in electrolyzed water of anode side is decreased; however, this proportion is not large.
Further at the anode, in addition to the reaction shown in the formula (13), H+ is present in slight excess owing to the difference in diffusion rate between ascorbic acid anion and H+; therefore, the amount of H+ is more than when only ascorbic acid is present. As a result, the dismutation ability for superoxide radicals is high at the anode side.
The above can be summarized as follows.
(1) When an aqueous electrolytic solution is electrolyzed which contains a water-soluble electrolyte (e.g. sodium chloride or potassium chloride) as an electrolytic aid and ascorbic acid as an additive, the electrolyzed water of cathode side has a dismutation activity but the action is low because ascorbic acid is consumed in the form of sodium or potassium ascorbate. The electrolyzed water of anode side contains hypochlorous acid, and this hypochlorous acid consumes the most part of ascorbic acid. Therefore, the electrolyzed water of anode side shows no dismutation activity.
(2) When an aqueous electrolytic solution containing only ascorbic acid as an electrolytic aid, at the cathode, the 2-position and 3-position hydrogen atoms of ascorbic acid are extracted by an electrophilic reaction, and ascorbic acid becomes dehydroascorbic acid. As a result, there is substantially no dismutation activity at the cathode side.
At the anode side, the ascorbic acid anion transferred from the cathode reacts with the H+ generated by anode oxidation of water and returns to ascorbic acid; therefore, there is a dismutation activity. Further, the dismutation activity of the electrolyzed water of anode side is enhanced by the presence of slightly excessive H+. The amount of ascorbic acid consumed by dissolved oxygen is extremely small.
In the present invention, an aqueous electrolytic solution containing only ascorbic acid as an electrolytic aid is electrolyzed; therefore, the electrolyzed water of anode side obtained has a high dismutation activity for superoxide radicals. Consequently, this electrolyzed water of anode side is useful in various applications such as sterilization, disinfection, granulation, maintenance of health or beauty, and the like. Further, since ascorbic acid is a vitamin which has been confirmed to be safe to human health, the electrolyzed water of anode side containing ascorbic acid, of the present invention has high safety.